Combined cycle power plant with improved efficiency

ABSTRACT

A CCPP includes a gas turbine, a HRSG, a steam turbine a flash tank and first and second supply lines. The gas turbine, the HRSG and the steam turbine are interconnected to generate power. The gas turbine may include an air preheating system to preheat the air supplied in the gas turbine to enable expedite combustion therein. The flash tank is fluidically connected at a cold end of the HRSG to extract waste hot water from the cold end. Further, the first supply line is configured to interconnect the flash tank and the steam turbine to supply of flash steam to the steam turbine. Furthermore, the second supply line is configured to interconnect the flash tank and the air preheating system to supply hot flash condensate thereto.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 14/523,085entitled “Combined Cycle Power Plant With Improved Efficiency” and filedOct. 24, 2014, which claims priority to European Application No.13191037.4 filed Oct. 31, 2013. Each of the foregoing applications ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to combined cycle power plants, and, moreparticularly, to an improvement in cycle arrangement for optimizing thecombined cycle power plant efficiency.

BACKGROUND

In a combined cycle power plant (CCPP), a gas turbine and a steamturbine combinedly produce electric power. The CCPP is arranged so thatthe gas turbine is thermally connected to the steam turbine via a heatrecovery steam generator (HRSG). The HRSG is a heat exchanger whichutilizes waste exhaust gas of the gas turbine to heat feedwater for thesteam generation process for operating the steam turbine. The optimumutilization of heat captured in the HRSG is one the major criteria toincrease CCPP efficiency.

Despite of various conventional techniques to capture the heat in HRSG,there may be chances that the heat, especially, at cold end of the HRSGmay be left unused. It therefore exists a need to optimally utilize theheat at the cold end of the HRSG in order to improve CCPP efficiency andoutput.

SUMMARY

The present disclosure describes an improvement in cycle arrangement foroptimizing the combined cycle power plant efficiency and output, thatwill be presented in the following simplified summary to provide a basicunderstanding of one or more aspects of the disclosure that are intendedto overcome the discussed drawbacks, but to include all advantagesthereof, along with providing some additional advantages. This summaryis not an extensive overview of the disclosure. It is intended toneither identify key or critical elements of the disclosure, nor todelineate the scope of the present disclosure. Rather, the sole purposeof this summary is to present some concepts of the disclosure, itsaspects and advantages in a simplified form as a prelude to the moredetailed description that is presented hereinafter.

An object of the present disclosure is to describe an improved combinedcycle power plant, which may be adaptable in terms of being improved incycle arrangement for optimizing the combined cycle power plantefficiency. Further, object of the present disclosure is to describe acombined cycle power plant, which is convenient to use in an effectiveand economical way. Various other objects and features of the presentdisclosure will be apparent from the following detailed description andclaims.

The above noted and other objects, in one aspect, may be achieved by animproved combined cycle power plant. The combined cycle power plantincludes at least one gas turbine, at least one heat recovery steamgenerator, at least one steam turbine, at least one flash tank, andfirst and second supply lines. The at least one gas turbine isconfigured to produce gas to generate power and subsequently releasingthe exhaust gas. The at least one gas turbine is configured to receiveheated air from a gas turbine air preheating system for enablingefficient combustion of fuel to produce the gas. Further, the at leastone heat recovery steam generator is thermally connected to the at leastone gas turbine to use heat energy extracted from the exhaust gas of theat least one gas turbine to produce steam. Furthermore, the at least onesteam turbine is thermally connected to the at least one heat recoverysteam generator to utilize the steam produced by the at least one heatrecovery steam generator to generate the power. The at least one flashtank fluidically connected at a cold end of the at least one heatrecovery steam generator to extract hot water from the cold end.Connection to the cold end of a heat recovery steam generator means thatthe flash tank is connected to the exit of a heat exchanger pipe or pipebundle which is arranged in the heat recovery steam generator closer tothe downstream (cold) end than to the upstream (hot) end. For examplethe e.g. heat exchanger pipe or pipe bundle is arranged in thedownstream half or downstream third of the heat recovery steamgenerator. Water is preheated in the cold end of the heat recovery steamgenerator relative to the extension of the heat recovery steam generatorin flow direction of the flue gases passing through it. Moreover, thefirst supply line is configured to interconnect the at least one flashtank and the at least one steam turbine to enable supply of flash steamto the at least one steam turbine. The second supply line is configuredto interconnect the at least one flash tank and the gas turbine airpreheating system to enable supply of hot flash condensate to the gasturbine air preheating system.

In one embodiment of the present disclosure, the combined cycle powerplant may include a feedwater preheating system thermally coupled to thecold end of the at least one heat recovery steam generator. Thismaintains the temperature requirement of feed water at the cold end ofthe heat recovery steam generator. In this embodiment, the hot waterused for flashing purposes may be extracted from the feedwaterpreheating system.

In one further embodiment of the present disclosure, the combined cyclepower plant may include a first bypass member to selectively bypass apart or all of the flash condensate from the second supply line forbypassing the gas turbine air preheating system.

In one further embodiment of the present disclosure, the combined cyclepower plant may include a second bypass member to selectively bypass apart or all of the extracted hot water from the cold end of the heatrecovery steam generator for bypassing the supply of the extracted hotwater in to the at least one flash tank, and to serve as a recirculationsystem for heating the condensate at the at least one heat recoverysteam generator to achieve a required minimum temperature.

In one further embodiment of the present disclosure, the combined cyclepower plant may include at least one additional heat source fluidicallyconnected the at least one flash tank to be utilized for additional hotwater extraction to be supplied to the at least one flash tank.

In one further embodiment of the present disclosure, the combined cyclepower plant may also include a provision for fuel gas preheating. Insuch embodiment, the combined cycle power plant may include a fuel gaspreheating system configured to heat fuel to be supplied to the at leastone gas turbine for effective combustion. In such configuration, thefuel gas preheating system is adapted to be configured in the secondsupply line to utilize the heat from the hot flash condensate to preheatthe fuel to be supplied to the at least one gas turbine.

In one further embodiment, the combined cycle power plant a plurality offlash tanks, instead of one flash tank, is provided. The plurality offlash tanks is configured for hot water extraction and subsequentlyinjecting the flash steam of varying pressure levels to a correspondingsteam turbine stages.

In one further embodiment, the combined cycle power plant may alsoinclude at least one heat exchanger adapted to be configured to thesecond supply line. In such embodiment, the at least one heat exchangeris configured with the gas turbine air preheating system for preheatingthe air supplied in the gas turbine engine.

In yet further embodiment of the combined cycle power plant, the atleast one gas turbine may preclude the gas turbine air preheatingsystem, where heating is not required. Rather in this embodiment, thesecond supply line is configured to supply hot flash condensate from theat least one flash tank to a return line to the at least one heatrecovery steam generator 120 or to the feedwater preheating system.

These together with the other aspects of the present disclosure, alongwith the various features of novelty that characterize the presentdisclosure, are pointed out with particularity in the presentdisclosure. For a better understanding of the present disclosure, itsoperating advantages, and its uses, reference should be made to theaccompanying drawings and descriptive matter in which there areillustrated exemplary embodiments of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the present disclosure will be betterunderstood with reference to the following detailed description andclaims taken in conjunction with the accompanying drawing, wherein likeelements are identified with like symbols, and in which:

FIG. 1 illustrates an example line diagram of a Combined Cycle Powerplant (CCPP) depicting a present invention incorporated therein, inaccordance with an exemplary embodiment of the present disclosure;

FIG. 2 illustrates another example embodiment depicting a first variantof the CCPP of FIG. 1;

FIG. 3 illustrates another example embodiment depicting a second variantof the CCPP of FIG. 1;

FIG. 4 illustrates another example embodiment depicting a third variantof the CCPP of FIG. 1;

FIG. 5 illustrates another example embodiment depicting a fourth variantof the CCPP of FIG. 1;

FIG. 6 illustrates another example embodiment depicting a fifth variantof the CCPP of FIG. 1;

FIG. 7 illustrates another example embodiment depicting a sixth variantof the CCPP of FIG. 1; and

FIG. 8 illustrates yet another example embodiment depicting a seventhvariant of the CCPP where a gas turbine precludes an air preheatingsystem.

Like reference numerals refer to like parts throughout the descriptionof several views of the drawings.

DETAILED DESCRIPTION

For a thorough understanding of the present disclosure, reference is tobe made to the following detailed description, including the appendedclaims, in connection with the above described drawings. In thefollowing description, for purposes of explanation, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present disclosure. It will be apparent, however, to one skilled inthe art that the present disclosure can be practiced without thesespecific details. In other instances, structures and apparatuses areshown in block diagrams form only, in order to avoid obscuring thedisclosure. Reference in this specification to “one embodiment,” “anembodiment,” “another embodiment,” “various embodiments,” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment of thepresent disclosure. The appearance of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, nor are separate or alternative embodimentsmutually exclusive of other embodiments. Moreover, various features aredescribed which may be exhibited by some embodiments and not by others.Similarly, various requirements are described which may be requirementsfor some embodiments but may not be of other embodiment's requirement.

Although the following description contains many specifics for thepurposes of illustration, anyone skilled in the art will appreciate thatmany variations and/or alterations to these details are within the scopeof the present disclosure. Similarly, although many of the features ofthe present disclosure are described in terms of each other, or inconjunction with each other, one skilled in the art will appreciate thatmany of these features can be provided independently of other features.Accordingly, this description of the present disclosure is set forthwithout any loss of generality to, and without imposing limitationsupon, the present disclosure. Further, the relative terms, such as“first,” and “second” and the like, herein do not denote any order,elevation or importance, but rather are used to distinguish one elementfrom another. Further, the terms “a” and “an” herein do not denote alimitation of quantity, but rather denote the presence of at least oneof the referenced item.

Referring now to FIGS. 1 to 7, examples of a Combined Cycle Power Plant(CCPP) 100 depicting various exemplary embodiments of the presentinvention incorporated therein. In as much as the construction andarrangement of the CCPP 100, various associated elements may bewell-known to those skilled in the art, it is not deemed necessary forpurposes of acquiring an understanding of the present disclosure thatthere be recited herein all of the constructional details andexplanation thereof. Rather, it is deemed sufficient to simply note thatas shown in FIGS. 1 to 7, the CCPP 100, only those components are shownthat are relevant for the description of various embodiments of thepresent disclosure.

Referring now to FIG. 1, the CCPP 100 for optimizing efficiency andoutput thereof includes at least one gas turbine 110, at least one heatrecovery steam generator (HRSG) 120, at least one steam turbine 130, atleast one flash tank 140 and first and second supply lines 150, 160. Asconventionally, the at least one gas turbine 110 is thermally conjoinedwith the at least one steam turbine 130 via the HRSG 120 to generate thepower output. Elaborately, as per present disclosure, the gas turbine110 is configured to produce gas to generate power and subsequentlyreleasing exhaust gas. The gas turbine 110 may include a gas turbine airpreheating system 112 for preheating the air and supplying the heatedair for enabling efficient combustion of the fuel to produce the gas inthe gas turbine 110. Further, the HRSG 120 is thermally connected to theat least one gas turbine 110 to use heat energy extracted from theexhaust gas of the at least one gas turbine 110 to produce steam.Furthermore, the at least one steam turbine 130 is thermally connectedto the HRSG 120 to utilize the steam to generate the power output.

As described, the HRSG 120 is a heat exchanger which utilizes wasteexhaust gas of the gas turbine 110 to produce steam for operating thesteam turbine. The optimum utilization of heat captured in the HRSG 120is one the major criteria to increase the CCPP 100 efficiency. However,despite of various conventional techniques to capture the heat in theHRSG 120, there may be chances that the heat, especially, at a cold end122 of the HRSG 120 may be left unused. the CCPP 100 of the presentinvention therefore includes the at least one flash tank 140 tooptimally utilize the heat at the cold end 122 of the HRSG 120 in orderto improve the CCPP 100 efficiency and power output. In as much as theconstruction and arrangement of the HRSG 120, various associatedelements may be well-known to those skilled in the art, and it is deemedsufficient to simply note that as shown in FIGS. 1 to 7, in the HRSG120, only those components are shown that are relevant for thedescription of various embodiments of the present disclosure.

As shown in FIG. 1, the flash tank 140 is fluidically connected, via asupply line 142, at the cold end 122 of the HRSG 120 to extract unusedhot water from the cold end 122. A flash valve 141 may be incorporatedto control extraction. In one embodiment of the present disclosure, theCCPP 100 may include a feedwater preheating system 170 thermally coupledto the cold end 122 of the HRSG 120 to maintain the temperaturerequirement of the feed water at the cold end 120. Also in suchembodiment, the flash tank 140 is fluidically connected at the cold end122 of the HRSG 120 through the feedwater preheating system 170 in orderto extract unused hot water of the cold end 122 via the feedwaterpreheating system 170. A recirculation system 172 may be enabled to thefeedwater preheating system 170 for cooperating thereto. Furthermore, aLow Pressure (LP) drum 126 may also be operative in conjunction to thefeedwater preheating system 170.

The flash tank 140 having the extracted hot water may direct therefromfor being utilized in the most optimized way in order to increase theefficiency of the CCPP 100. For the said purpose, the CCPP 100 includesthe first and second supply lines 150, 160.

The first supply line 150 is configured to interconnect the flash tank140 and the steam turbine 130 to enable supply of flash steam thereto.The flash steam from the first supply line 150 may be injected to thesteam turbine 130, more specifically, to a Low Pressure (LP) steamturbine, to generate an additional power, apart from the power generatedin conventional CCPP cycle, in order to increase the efficiency andoutput of the CCPP 100 and thereafter is supplied back to the HRSG 120via a return line 136 through a pump 137 via passing through a condenser135.

Further, the second supply line 160 is configured to interconnect theflash tank 140 and the gas turbine air preheating system 112 to enablesupply of hot flash condensate in the flash tank 140 to the gas turbineair preheating system 112 to preheat the air to supply the preheated airto the gas turbine 110 for enabling expedite combustion. A pump 162 maybe configured in the second supply line 160 to achieve the target.Further, the cold condensate therefrom may be returned to the returnline 136 and resupplied to the HRSG 120 for reoperation. The secondsupply line 160 enables the provision of preheating the air supplied inthe gas turbine 110, thereby replacing the requirement of externalsource to preheat the fuel air in turn increasing the efficiency of theCCPP 100. Further, the said provision of air preheating of the presentinvention precludes complex piping and heat exchanger arrangements asrequired in the conventional CCPP, making the present system simple andcost effective.

The CCPP 100 as depicted in FIG. 1, as per various embodiments, may bemodified to different plant configurations. FIGS. 2 to 7 depict varioussuch variants of the CCPP 100, and will be described herein inconjunction to FIG. 1. In FIGS. 2 to 7 only those reference numeralswith corresponding components are inserted where the variant aredepicted, and the remaining components are left unnumbered in order tohighlight the various variant, and that the all unnumbered componentsshall be read or referred from FIG. 1.

As per the first variant, as shown in FIG. 2, the CCPP 100 may include afirst bypass member 180 to selectively bypass a part or all of the flashcondensate from the second supply line 160 for bypassing the gas turbineair preheating system 112.

As per the second variant, as shown in FIG. 3, the CCPP 100 may includea second bypass member 190 to selectively bypass a part or all of theextracted hot water from the cold end 122 for bypassing the supply ofthe extracted hot water in to the flash tank 140. This serve as arecirculation system for heating the condensate to the HRSG 120 toachieve a required minimum HRSG inlet temperature. In such arrangement,the recirculation system 172 may be precluded, as shown in FIG. 3.

As per the third variant, as shown in FIG. 4, the CCPP 100 may includeat least one additional heat source 200 fluidically connected the flashtank 140 to be utilized for additional hot water extraction to besupplied to the flash tank 140. The additional heat source 200 mayinclude condensate return from a carbon capture unit or a districtheating unit. However, without departing from the scope of the presentdisclosure, the additional heating source may be any other as known inthe art.

As per the fourth variant, as shown in FIG. 5, the CCPP 100 may includea fuel gas preheating system 300 configured to heat fuel to be suppliedto the gas turbine 110 for effective combustion. The fuel gas preheatingsystem 300 is adapted to be configured in the second supply line 160 toutilize the heat from the hot flash condensate to preheat the fuel to besupplied to the gas turbine 110. For example, at least one heatexchanger, such as 310, may be configured in series with a heatexchanger 320 of the second supply line 160 for enabling the heat to besupplied to preheat the fuel gas at the fuel gas preheating system 300as and when required full temperature, normally 150-300° C., is notgenerated by the hot flash condensate of the second supply line 160.

In one other variant, fifth, as shown in FIG. 6, the CCPP 100 mayinclude a plurality of flash tanks 140 as against one flash tank 140.The plurality of flash tanks 140 is adapted to work in cascadingconfiguration, using as hot source the hot water from the cold end 122of the HRSG 120, flashing at different pressures and subsequentlyinjecting the flash steam of varying pressure levels to correspondingsteam turbine stages

In one further variant, sixth, as shown in FIG. 7, at least one heatexchanger 400 adapted to be configured to the second supply line 160. Asshown, the at least one heat exchanger 400 is configured in anintermediate heat transfer loop 420 with the gas turbine air preheatingsystem 112 for preheating the air supplied in the gas turbine 110.

In yet another variant, seventh, as shown in FIG. 8, of the CCPP 100,the at least one gas turbine 110 may preclude the gas turbine airpreheating system 112 as provided in the above embodiments. Rather inthis embodiment, the second supply line 160 is configured to supply hotflash condensate from the at least one flash tank 140 to the return line136 to the at least one heat recovery steam generator 120 or to thefeedwater preheating system 170. Valves 141 may be configured to controlrespective supply of the hot water and condensate.

The invention of the present disclosure is advantageous in variousscopes. This provides an improved CCPP, which may capture the heat inthe HRSG, especially, at the cold end of the HRSG that may be leftunused in most optimal way in order to improve CCPP efficiency andoutput. Further, the CCPP with such variants is convenient to use andeconomical. Various other advantages and features of the presentdisclosure are apparent from the above detailed description andappendage claims.

The foregoing descriptions of specific embodiments of the presentdisclosure have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit thepresent disclosure to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the present disclosure and its practicalapplication, to thereby enable others skilled in the art to best utilizethe present disclosure and various embodiments with variousmodifications as are suited to the particular use contemplated. It isunderstood that various omission and substitutions of equivalents arecontemplated as circumstance may suggest or render expedient, but suchare intended to cover the application or implementation withoutdeparting from the spirit or scope of the claims of the presentdisclosure.

What is claimed is:
 1. A combined cycle power plant, comprising: atleast one gas turbine configured to produce gas to generate power andsubsequently releasing exhaust gas, the at least one gas turbineconfigured to receive heated air from a gas turbine air preheatingsystem for enabling efficient combustion of fuel to produce the exhaustgas; at least one heat recovery steam generator thermally connected tothe at least one gas turbine to use heat energy extracted from theexhaust gas of the at least one gas turbine to produce steam, the atleast one heat recovery steam generator comprising a heat exchangerdisposed within an exhaust flow path through the at least one heatrecovery steam generator; at least one steam turbine thermally connectedto the at least one heat recovery steam generator to utilize the steamproduced by the at least one heat recovery steam generator to generatethe power; at least one flash tank directly coupled to a cold end of theheat exchanger of the at least one heat recovery steam generator by aconduit, wherein the cold end of the heat exchanger is disposed withinthe exhaust flow path through the at least one heat recovery steamgenerator, and the at least one flash tank extracts hot water from thecold end of the heat exchanger through the conduit; a first supply lineinterconnecting the at least one flash tank and the at least one steamturbine to enable supply of flash steam to the at least one steamturbine from the hot water extracted from the heat exchanger of the atleast one heat recovery steam generator; a second supply line directlyinterconnecting the at least one flash tank to both the gas turbine airpreheating system and a bypass line to selectively enable a bypassing ofpart or all flash condensate supplied to the gas turbine air preheatingsystem by the at least one flash tank; a first return line extendingfrom the at least one steam turbine to the cold end of the heatexchanger of the at least one heat recovery steam generator, wherein thefirst return line is configured to enable flow of the flash steam fromthe at least one steam turbine toward the cold end of the heat exchangerof the at least one heat recovery steam generator; and a second returnline interconnecting the gas turbine air preheating system and the firstreturn line, wherein the second return line is configured to enable flowof the flash condensate from the gas turbine air preheating system tothe first return line, and a junction between the second return line andthe first return line is positioned upstream of the at least one flashtank.
 2. The combined cycle power plant of claim 1, further comprising afeedwater preheating system thermally coupled to the cold end of theheat exchanger, wherein the conduit further comprises at least one flashvalve disposed therein.
 3. The combined cycle power plant of claim 1,wherein the at least one flash tank is a single flash tank disposedalong a fluid flow path between the cold end of the heat exchanger andthe at least one steam turbine, the fluid flow path comprises theconduit directly connecting the cold end of the heat exchanger and thesingle flash tank and the first supply line directly connecting thesingle flash tank and the at least one steam turbine.
 4. The combinedcycle power plant of claim 3, wherein the hot water received by thesingle flash tank is heated only by the exhaust gas in the at least oneheat recovery steam generator.
 5. The combined cycle power plant ofclaim 1, comprising a feedwater preheating system, wherein the feedwaterpreheating system is thermally coupled to a last section of the heatexchanger in a direction of flow along the exhaust flow path through theat least one heat recovery steam generator.
 6. A combined cycle powerplant, comprising: at least one gas turbine configured to produce gas togenerate power and subsequently releasing exhaust gas; at least one heatrecovery steam generator thermally connected to the at least one gasturbine to use heat energy extracted from the exhaust gas of the atleast one gas turbine to produce steam, the at least one heat recoverysteam generator comprising a heat exchanger disposed within an exhaustflow path through the at least one heat recovery steam generator; atleast one steam turbine thermally connected to the at least one heatrecovery steam generator to utilize the steam produced by the at leastone heat recovery steam generator to generate the power; a single flashtank having a fluid connection to a cold end of the heat exchanger ofthe at least one heat recovery steam generator, wherein the cold end ofthe heat exchanger is disposed within the exhaust flow path through theat least one heat recovery steam generator, the single flash tankextracting hot water from the cold end of the heat exchanger through thefluid connection, and the hot water is heated only by the exhaust gasflowing along the exhaust flow path; a first supply line interconnectingthe single flash tank and the at least one steam turbine to enablesupply of flash steam to the at least one steam turbine from the hotwater extracted from the heat exchanger of the at least one heatrecovery steam generator; a second supply line directly interconnectingthe single flash tank to both a gas turbine air preheating system and abypass line to selectively enable a bypassing of part or all flashcondensate supplied to the gas turbine air preheating system by thesingle flash tank; and a first return line extending from the at leastone steam turbine to the cold end of the heat exchanger of the at leastone heat recovery steam generator, wherein the first return line isconfigured to enable flow of the flash steam from the at least one steamturbine toward the cold end of the heat exchanger of the at least oneheat recovery steam generator; and a second return line interconnectingthe gas turbine air preheating system and the first return line, whereinthe second return line is configured to enable flow of the flashcondensate from the gas turbine air preheating system to the firstreturn line, and a junction between the second return line and the firstreturn line is positioned upstream of the single flash tank.
 7. Thecombined cycle power plant of claim 6, wherein the fluid connection isfluidly connected to at least one LP drum.
 8. The combined cycle powerplant of claim 6, further comprising at least one condenser fluidlycoupled downstream of the at least one steam turbine.
 9. The combinedcycle power plant of claim 8, further comprising at least one first pumpfluidly coupled downstream of the at least one condenser.
 10. Thecombined cycle power plant of claim 9, further comprising at least onesecond pump in fluid communication with the second supply line, the atleast one second pump fluidly coupled downstream of the single flashtank, wherein the at least one second pump is fluidly coupled upstreamof the bypass line.
 11. The combined cycle power plant of claim 6,wherein the fluid connection comprises a conduit directly connecting thecold end of the heat exchanger and the single flash tank, and the firstsupply line directly connects the single flash tank and the at least onesteam turbine.
 12. The combined cycle power plant of claim 6, comprisinga feedwater preheating system, wherein the feedwater preheating systemis thermally coupled to the cold end of the heat exchanger.
 13. Thecombined cycle power plant of claim 6, wherein the cold end is a lastsection of the heat exchanger in a direction of flow along the exhaustflow path through the at least one heat recovery steam generator.
 14. Asystem, comprising: a heat recovery steam generator (HRSG) configured torecover heat from an exhaust gas to generate steam, wherein the HRSGcomprises a plurality of heat exchangers disposed within the exhaustflow path at different positions in a direction of flow along theexhaust flow path, and a last heat exchanger of the plurality of heatexchangers in the direction of flow is configured to transfer heat fromthe exhaust gas into water to produce a heated water; a steam turbinethermally connected to the HRSG; a flash tank fluidly coupled to thelast heat exchanger in the HRSG, wherein the flash tank is configured toreceive the heated water from the last heat exchanger, the heated wateris heated only by the exhaust gas in the HRSG, and the flash tank isconfigured to flash the heated water to supply a flash steam to thesteam turbine; a first supply line interconnecting the flash tank andthe steam turbine to enable flow of the flash steam from the flash tankto the steam turbine; a second supply line interconnecting the flashtank and a gas turbine air preheating system to enable flash condensateto flow from the flash tank to the gas turbine air preheating system; afirst return line extending from the steam turbine to the last heatexchanger in the HRSG, wherein the first return line is configured toenable flow of the flash steam from the steam turbine toward the lastheat exchanger in the HRSG; and a second return line interconnecting thegas turbine air preheating system and the first return line, wherein thesecond return line is configured to enable flow of the flash condensatefrom the gas turbine air preheating system to the first return line, anda junction between the second return line and the first return line ispositioned upstream of the flash tank.
 15. The system of claim 14,wherein only the one flash tank is disposed along a fluid flow pathbetween the last heat exchanger and the steam turbine, and the fluidflow path comprises a heated water conduit extending between the lastheat exchanger and the flash tank.
 16. The system of claim 14, wherein aheated water conduit directly couples the flash tank to the last heatexchanger.
 17. The system of claim 14, comprising a gas turbineconfigured to output the exhaust gas to the HRSG.